1. Field of the Invention
The present invention relates to a photoelectric conversion device for forming an image by visible light, radiation, or the like and, more particularly, to a photoelectric conversion device suitably applicable to a two-dimensional photoelectric conversion apparatus such as a still camera or an X-ray image pickup system.
2. Related Background Art
The conventional photography was mostly the silver salt photography using an optical camera and silver salt film. The development of the semiconductor technology has brought about development of an image pickup apparatus capable of taking images of moving picture, such as video camcorders, by a solid image pickup device using Si single-crystal sensors typified by CCD sensors or MOS sensors, but these images were inferior in the number of pixels and in S/N ratios to silver-salt photographs. Therefore, it was common practice to use the silver salt photography for taking still images with high quality.
On the other hand, demands are increasing in recent years for image processing by computer, storage of image in the form of an electronic file, and transmission of image by electronic mail. Under such circumstances, there are desires for electronic image pickup device capable of outputting a digital signal of an image comparable to those of the silver salt photography. This is not the case only in the field of ordinary photographs but also in the field of medical care.
The X-ray photography is popularly known as application of the silver salt photography technology in the field of medical care. This is the photography for exposing the affected part of a human body to X-rays emitted from an X-ray source and, for example, for determining whether a fracture or a tumor is present, based on information of X-ray transmission, which has been and is widely used long in medical diagnosis. Normally, X-rays transmitted by the affected part are made incident once to a fluorescent member to be converted into visible light; and the silver salt film is exposed to this visible light. The silver salt film has advantages of high sensitivity and high resolution, but also has disadvantages of taking some time for development, requiring much time and labor for storage and management, not allowing quick transmission of data to a remote place, and so on. There are thus desires for electronic X-ray image pickup device capable of outputting a digital signal of an image equivalent to those of the silver salt photography, as stated above. A suggestion to implement it was a method for forming an image by using a reducing optical system and a compact photoelectric conversion device using a single-crystal, such as the CCD sensors or the MOS sensors.
This method was, however, able to utilize only about one thousandth of light emitted from the fluorescent member and thus was still susceptible to improvement against the requirement that diagnosis should be made with as weak X-rays as possible where the human body was observed with X-rays. It is thus not easy to implement an X-ray diagnosis device for medical care by the compact photoelectric conversion device using the reducing optical system of poor light utilization efficiency.
In order to meet this requirement, development is under way of an image pickup apparatus using a large sensor with a two-dimensional array of image pickup devices using photoelectric conversion elements having hydrogenated amorphous silicon (hereinafter referred to as xe2x80x9ca-Sixe2x80x9d). The image pickup device of this type is constructed in such structure that a metal and a-Si are deposited in a desired order on an insulating substrate having each side of 30 to 50 cm by a sputtering apparatus, a chemical vapor deposition apparatus (CVD apparatus) or the like, for example, approximately 2000xc3x972000 semiconductor diodes are formed therein, an electric field of a reverse bias is applied thereto, and charges flowing in the reverse direction in the respective diodes can be individually detected by a thin film transistor (hereinafter referred to as xe2x80x9cTFTxe2x80x9d) made at the same time as the diodes. It is popularly known that when the electric field of the reverse direction is applied to the semiconductor diode, a photocurrent flows corresponding to a quantity of light incident to the semiconductor layer. The above device utilizes this phenomenon. However, a current, so called a dark current, flows even in a state in which no light is present at all and this gives rise to shot noise, which is the cause of degradation of detection capability of the overall system, i.e., degradation of the sensitivity called the S/N ratio. The point of development is thus how much this dark current can be decreased.
EP-A-0660421 discloses structural examples of X-ray image pickup systems for satisfying these requirements.
FIG. 1 is a schematic circuit diagram of a photoelectric conversion device used in an X-ray image pickup system. FIG. 2 is a schematic block diagram of another X-ray image pickup system.
The above image pickup systems were, however, still susceptible to improvement against the requirements of higher S/N, better operability, and lower cost. The reasons will be described below with the examples of the image pickup systems of FIG. 1 and FIG. 2.
In FIG. 1, S11, S12, . . . , Smn (m and n are positive integers) represent photosensors, T11, T12, . . . , Tmn (m and n are positive integers) switching elements such as thin film transistors, C11, C12, . . . , Cmn (m and n are positive integers) capacitor elements, and SR1 and SR2 shift registers. One pixel is composed of a photosensor S1, . . . , or Smn, a capacitor element C11, . . . , or Cmn, and a switching element T11, . . . , or Tmn, and the pixels are arranged in a matrix pattern. The switching element T11, . . . , or Tmn of each pixel is used for transmission of signal. Gates of the respective switching elements T11, . . . , Tmn of pixels in each row are connected to a control line g1, g2, . . . , or gm (m is a positive integer), and the control lines g1, . . . , gm are connected to the shift register SR1. Each one main electrode of the respective switching elements T11, . . . , Tmn of pixels in each column are connected to each signal line provided for every column. One electrode of each of the photosensors S11, . . . , Smn and one electrode of each of capacitor elements C11, . . . , Cmn in each pixel are connected in common and then connected to a switch SWg and a switch SWs. The other electrode of each of the photosensors S11, . . . , Smn and the other electrode of each of capacitor elements C11, . . . , Cmn in each pixel are connected to the other main electrode than the above one main electrode of each switching device T11, . . . , Tmn. Each signal line is connected to a switch M1, M2, . . . , or Mn (n is a positive integer), and the switches M1, . . . , Mn are successively driven by the shift register SR2 to output signals as outputs in order through an amplifier. Switches SWg and SWs are connected to desired power supplies Vg and Vs, respectively, and are driven so as to give a desired potential to the one electrodes of each of the photosensors S11, . . . , Smn and the one electrode of each of capacitor elements C11, . . . , Cmn at desired timing.
In FIG. 2, numeral 6001 indicates a photoelectric conversion portion, 6002 an analog/digital signal converter for converting analog signals from the photoelectric conversion portion 6001 to digital signals, 6003 subtracters for correction of fixed pattern, 6004 a memory, 6005 a controller, 6006 a buffer, and 6007 an image processor. FIG. 2 shows a example in which a plurality of shift registers SR1 are arranged in series and a plurality of integrated circuits IC for detection are arranged. Outputs from the integrated circuits IC for detection are input to the analog-digital signal converters 6002 in the processing circuit 6008 to be digitized. Each digital output is supplied to the subtracter 6003 for correction of fixed pattern to remove unwanted fixed pattern noise therefrom, and then is stored in each memory 6004. The information stored in the memories 6004 is controlled by the controller 6005, and is transferred through the buffer 6006 to the image processor 6007 for signal processing to undergo image processing.
As the first problem, when pieces of information of nxc3x97m photoelectric conversion elements are obtained from the photoelectric conversion device in which m lines of n photoelectric conversion elements on each line are arranged, as in the example of FIG. 1, if n and m are not less than 1000, the operational speed of the A/D converter is not sufficient.
Although the A/D converter is not illustrated in FIG. 1, it is common practice to connect one A/D converter to Vout to convert an analog voltage to digital information. In order to obtain the digitized information from the information from the photoelectric conversion elements in this structure, some time is necessary for the A/D converter to convert the analog voltage outputted to Vout to the digital information. When Tad is defined as a time necessary for the A/D converter to obtain the digital information, the device needs the following time T(1 line) for obtaining n pieces of digital information from the pixels on one line, i.e., T(1 line)xe2x89xa7nxc3x97Tad. In practice, the device further requires a time for turning the transferring TFTs Tx1 to Txn on and a time for successively turning the switches M1 to Mn on. Therefore, a furthermore time is necessary.
The device requires the following time T(1 frame) for obtaining information of one frame, i.e., T(1 frame)xe2x89xa7mxc3x97nxc3x97Tad. If n=m=2000, at least the time of 4,000,000xc3x97Tad is necessary for obtaining information of one frame. Since the time for analog-to-digital conversion of the A/D converter is normally 100 nsec to 1000 nsec, 0.4 second to 4 seconds are necessary for obtaining the information of one frame after all. This time is desired to be decreased in view of the desires for increase of S/N ratios and improvement in operability. This is because an accumulation time of dark current becomes long. Reading is started after completion of exposure to the photoelectric conversion elements. Supposing the reading takes four seconds, the dark current flowing during the period of four seconds would be accumulated in the photoelectric conversion element read last. This accumulation time of dark current is too long even with use of the photoelectric conversion elements having a small dark current described above, thereby giving rise to the shot noise. This will be the cause of degrading the detection capability of the overall apparatus, i.e., the cause of decrease of S/N ratios. When the reading takes four seconds, a reading takes at least four seconds. In that case, a patient must stop his breath with determination to stand still for four seconds or more. Therefore, improvement is demanded in terms of the operability.
A system using signal wires SIG divided to some groups and a plurality of A/D converters in order to improve this point is the system illustrated in FIG. 2. European Patent No. 0440282 discloses an device similar to such a system. These systems are, however, susceptible to solving the second and third problems described below.
The second problem is that in FIG. 1, before the switches M1 to Mn are successively turned on, the transferring TFTs Tx1 to Txn (x is a number selected from integers from 1 to m) have to be turned on to stabilize the potentials of the signal wires SIG. Since the A/D converter must convert an analog voltage to a digital signal during a period in which one switch My (y is a number selected from integers from 1 to n) is open, a time TM necessary for successively turning the switches M1 to Mn on is TMxe2x89xa7nxc3x97Tad. In practice, a more time is necessary, because the A/D converter is unoperatable during the period after switching from the switch My to the switch M(y+1) and before stabilization of the potential of Vout. This problem can be relaxed by using a plurality of A/D converters like the system of FIG. 2 described above and the device disclosed in European Patent No. 0440282. It is, however, necessary that the transferring TFTs Tx1 to Txn be turned on to stabilize the potentials of the signal wires SIG during the period between acquisition of digital information of one line and acquisition of digital information of a next line. When this time is defined as Ttft, the time T(1 line) for acquiring n pieces of digital information on one line is given as T(1 line)xe2x89xa7TM+Ttft.
The third problem is that, though the switches M1 to Mn are ideally to be turned on in order after the transferring TFTs Tx1 to Txn have been turned on to stabilize the potentials of the signal wires SIG, a small leakage current flows in the signal wires SIG in practice to decrease the signal charge during the successive switching-on operation of the switches M1 to Mn or to add an additional charge to the original signal, thereby lowering the S/N ratios. The transferring TFTs have a certain resistance even in an on state (so called xe2x80x9con resistancexe2x80x9d), and this could make movement of signal charge instable. The decrease of S/N ratios is likely to occur when the time t is long from the turning on of the transferring TFTs Tx1 to Txn to the moment of the analog-to-digital conversion of information in the A/D converter through the switch My. If this time t is too short conversely, the on resistances of the transferring TFTs could also lower the S/N ratios. This means that there is a desired value of this time t for obtaining high S/N ratios.
On the other hand, in the method for successively turning the switches M1 to Mn on and converting information to digital data by the A/D converter through the switches My, the time t differs depending upon each of the photoelectric conversion elements Sx1 to Sxn. Specifically, the photoelectric conversion element Sx1 has a short time t from the turning on of the transferring TFT Tx1 to Txn to the moment of the analog-to-digital conversion of information by the A/D converter through the switch M1, whereas the photoelectric conversion element Sxn has a long time t from the turning on of the transferring TFTs Tx1 to Txn to the moment of the analog-to-digital conversion of information by the A/D converter through the switch Mn. This may cause such cases that information cannot be obtained in the desired time t for all the photoelectric conversion elements. This third problem is not a problem only in the device illustrated in FIG. 1, but may also be a problem arisen in such device that elements such as amplifiers exist before the switch group or so-called analog multiplexer, like the device shown in European Patent No. 0440282.
A solution to the first to third problems described above is a configuration in which n A/D converters are provided, the transferring TFTs Tx1 to Txn are turned on without using the switches My, and all the A/D converters are operated after a lapse of the desired time t to convert information to digital data. This is, however, difficult in practice where n is large, e.g., not less than 1000. Even if such a configuration can be implemented, a lot of expensive A/D converters will be used and thus raise the cost.
An object of the present invention is to provide a low-cost photoelectric conversion device with high S/N ratios and with good operability and a low-cost system capable of obtaining digital information with a large area and high S/N ratios necessary for the X-ray image pickup system or the like.
Another object of the present invention is to provide a photoelectric conversion device comprising:
a photoelectric conversion portion comprising a plurality of photoelectric conversion pixels arranged in row and column directions, a plurality of signal wires wired in the column direction, each of signal wires connecting outputs of the photoelectric conversion pixels in one and the same column, and a plurality of control lines wired in the row direction, each of the control lines connecting control terminals for controlling signal output operation of the photoelectric conversion pixels arranged in one and the same row;
a plurality of analog memory means for storing an analog voltage obtained from analog voltage conversion means for converting an information charge based on the photoelectric conversion pixels to the analog voltage, each of the analog memory means being connected to each of the signal wires; and
a plurality of A/D conversion means each connected to each output of a plurality of analog switch means, each of the analog switch means being connected to each of a plurality of output line groups which are formed by dividing output lines of each of the analog memory means into a plurality of groups.
Another object of the present invention is to provide the above photoelectric conversion device wherein the analog voltage converting means and the analog memory means are connected to each of the signal wires.
Another object of the present invention is to provide the above photoelectric conversion device wherein when the plurality of output line groups are composed of N groups of output lines, the photoelectric conversion pixels are arranged in n columns, a conversion time of the A/D conversion means is Tad second, and a time for conversion of the information charge outputted from the photoelectric conversion pixels to the analog voltage through the analog voltage conversion means is Ttft second,
the N satisfies the following condition:
Nxe2x89xa7nxc3x97Tad/Ttft.
Another object of the present invention is to provide the above photoelectric conversion device wherein the N satisfies the condition of nxc3x97Tad/Ttftxe2x89xa6N less than nxc3x97Tad/Ttft+1.
Another object of the present invention is to provide the above photoelectric conversion device wherein the photoelectric conversion pixel has a photoelectric conversion element and a switching element for controlling the signal output operation of the photoelectric conversion element and wherein a control terminal of the photoelectric conversion pixel is a control terminal of the switching element.